Organic Gardening : Soil Solarization for Weed Control

By J. E. Davidson

Organic home gardening has become increasingly popular as we become more aware of how toxic chemicals can harm our health and the health of our environment. Soil solarization is an organic method of keeping weeds from taking over your gardening efforts. Prevent weeds before they even have a chance to grow! Begin your soil solarization in June or July (in the northern hemisphere). During the summer months the heat of the sun will "cook" the weeds for you and also destroy disease-causing plant pathogens in the top few inches of the soil.

Mow the weeds down as low as possible in the area you wish to garden and till the ground thoroughly to a depth of six to eight inches. Use a steel rake to comb out the roots you have turned up. Rake the surface again to smooth it out and water the ground. Sprinklers or drip irrigation work best because you want the soil wet, but you don't want standing puddles. Install a drip irrigation system at this point, if you want one, because you don't want to disturb the soil any more than necessary after the process is finished.

Weed seeds you can't see will be hiding in the soil waiting for a chance to sprout. Dry seeds are heat-resistant and moistening them will make them vulnerable to the high temperature that solarization creates. Particularly vigorous weeds may find their way through landscape fabric and mulch, so you must kill the seeds before they sprout.

Cover the moistened area with clear polyethylene or PVC plastic sheets that are one or two mils thick. Stretch the plastic tightly and weigh it down with bricks to keep it from blowing away. Adding a second plastic sheet will increase the effectiveness of the method by creating a mini-greenhouse effect. Use objects such as bricks or soda cans to create an air gap between the layers of plastic. Leave the plastic sheets on the area for at least four to six weeks and let the sun do its work!

Once you remove the plastic cover don't till the soil again or you may bring weed seeds still deep in the soil back to the surface. Landscape fabric will allow necessary water, air, and nutrients to seep through while discouraging weeds. Some gardeners use black plastic as a landscape fabric, but this will create an environment for harmful bacteria, fungus, and mold to grow. Fasten the landscape fabric down with staple-like pins that are made for this purpose. You don't need many pins, since you will be adding mulch which will help hold the fabric down. Place a pin about every 10 feet along the exposed edges and seams.

Cracks and opening in the landscape fabric give weeds an opportunity to invade your garden again. When figuring the amount you need keep in mind that the fabric should have a three to six inch overlap where the edges meet. A good quality landscape fabric should be fairly stiff and should not stretch or tear. The fabric comes in different thicknesses but the one that weigh three ounces per square yard are usually as effective as heavier weights and will cost less. Choose a name brand landscaping fabric; store brands and off-brands are often low in quality. It may be a bit of an initial expense, but when the fabric is completely covered with mulch, which protects it from the sun, it can last for years.

Once you have covered the garden with landscape fabric wait a few days before planting to allow the soil to cool down. Cut an X-shaped incision in the fabric in the spots where you want plants. Make the cuts only large enough to fit the root ball through, and don't cut circles or holes in the fabric. Fold the flaps back and place the plant in the ground. Try not to scatter much soil on top of the fabric. Once the plants are in place, fold the flaps around them and add mulch.

You may choose not to use landscape fabric, but it will keep your mulch from decomposing as quickly and adds an additional barrier to prevent weed growth. Use weed-free organic mulch. A few airborne weed seeds may settle in your mulch, but they will be easy to pull since the mulch is much looser than garden soil and the roots won't be firmly entrenched.

It won't hurt to leave the plastic sheets on your garden plot for longer than a month or two; long periods of heating will kill persistent weeds like crabgrass and nutsedge. If you are not going to plant right away the plastic will increase the effectiveness of the soil solarization and prevent airborne seeds from contaminating your garden plot.